Semiconductor device and method of manufacturing semiconductor device

ABSTRACT

A semiconductor device includes a semiconductor layer stacked on a substrate, a stripe-shaped ridge formed on a surface of the semiconductor layer, and electrode formed on an upper surface of the ridge and a protective film disposed on each side of the ridge. The electrode includes a flat portion having a flat surface substantially parallel to the upper surface of the ridge and sloped portions on both sides of the flat portion with each of the sloped portions having a sloped surface that is sloped with respect to the upper surface of the ridge. The protective film covers a region from a side surface of the ridge to the sloped surface of the sloped portion of the electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2009-134281 filed on Jun. 3, 2009. The entire disclosure of JapanesePatent Application No. 2009-134281 is hereby incorporated herein byreference.

BACKGROUND

1. Technical Field

The present invention relates to a semiconductor device and a method ofmanufacturing the semiconductor device, and particularly to asemiconductor device having a ridge and a method of manufacturing thesemiconductor device.

2. Related Art

There has been proposed is a compound semiconductor device in which astripe-shaped ridge is formed on a surface of a p-side semiconductorlayer of a compound semiconductor device, and a part of an active layerbelow the ridge is designated as an optical waveguide region. In such acompound semiconductor device, generally, a stripe-shaped ridge isformed on a surface of a compound semiconductor layer stacked on asubstrate, and an electrode is electrically connected on thestripe-shaped ridge.

Typical examples of such a compound semiconductor include a group III-Vcompound semiconductor, in which, a compound semiconductor having adesired composition ratio can be obtained by using one or a plurality ofgroup III elements or group V elements. Among those, as a semiconductorlaser capable of emitting light from ultraviolet region to visible lightregion including green region, a semiconductor laser using a nitridesemiconductor such as InAlGaN has been extensively studied.

For example, in a case where a material such as GaAs is used, due to itslow contact resistivity, its effects on the laser operatingcharacteristics is hardly expected even if the contact area between theupper surface of the stripe-shaped ridge and the electrode is changed,and a meaningful increase in laser operating voltage is unlikely tooccur. Whereas, in a case where a material made of a nitridesemiconductor such as GaN is used, due to its higher contact resistivitycompared to that of GaAs, the contact resistivity between the electrodeand the upper surface of the ridge may be increased according to achange in the contact area between the upper surface of the ridge andthe electrode, which may results in increase in laser operating voltage.

In a case where the width of the ridge is increased in order to preventincrease in the contact resistance between the electrode and the uppersurface of the ridge, the resulting laser beam will be a multimode laserbeam. Even in an application in which a semiconductor device capable ofemitting multimode laser beam can be used, the area of the electrode isincreased by increasing the ridge width, so that absorption of laserbeam at the electrode may generate a problem. An increase in loss due toabsorption of laser beam at the electrode as described above may resultin decrease in the slope efficiency.

JP 2004-22989A discloses formation of an upper electrode layer and aridge stripe, in which, an upper electrode layer (10) is formed on astacked layer structure (100) of a nitride gallium-based compoundsemiconductor, and a photoresist (40) for stripe is disposed thereon,then, the upper electrode layer and the ridge stripe is formed by usingthe photoresist as a mask.

JP 2004-119772A discloses formation of a ridge 109a, in which a stackedlayer pattern of a SiO₂ layer 4 and a ZrO₂ layer 5, and the ridge 109ais formed by way of dry etching using the ZrO₂ layer 5 as a mask. Next,after depositing a ZrO₂ film on the entire surface, the stacked layerpattern described above is used as a mask for liftoff to selectivelyleave the ZrO₂ film 7a at the both sides of the ridge 109a.

JP 2008-98349A discloses formation of a ridge, in which a stacked layermask part which is made of three layers and has a stripe-shaped patternis disposed and a ridge part is formed using the stacked layer mask partas a mask. Next, only the mask part at the second sub-layer of thestacked layer mask part is etched from the side surface to form a neckpart in the stacked layer mask part, then an insulating film isvapor-deposited on the entire upper surface of this condition. Next, thestacked layer mask part is dissolved to liftoff the insulating layerdisposed on the surface of the stacked layer mask part, and an openingof the insulating layer is defined in the upper surface of the ridgepart.

SUMMARY

JP 2004-22989A features reduction of variation in the width of theelectric current injection region, and JP 2004-119772A features precisecontrol of the width of a protruded portion. JP 2008-98349 featurescontrolling variation in driving current and/or driving voltage of alaser device by preventing an insulating material from depositing on theupper surface of the ridge.

These methods of manufacturing described above concern a means ofcontrolling the width of ridge and/or electrode on the ridge, but do notconcern about preventing absorption of laser beam by the electrodeformed on the ridge, and thus, they are still subjected to the problemsdescribed above.

Reduction in slope efficiency occurs when an electrode on the ridgeabsorbs the emitted laser beam. Accordingly, a need arises for asemiconductor device which can control the contact resistance betweenthe nitride semiconductor and the electrode at the upper surface of theridge, and which can prevent light absorption by the electrode on theridge.

Accordingly, the present invention is conceived in view of theabove-described problems, and an object of the present invention is toprovide a semiconductor device and a method of manufacturing thesemiconductor device, which is capable of preventing light absorption byan electrode and has improved reliability, while realizing a simplifiedmethod of manufacturing.

A semiconductor device according to the first aspect of the presentinvention includes a semiconductor layer stacked on a substrate, astripe-shaped ridge formed on a surface of the semiconductor layer, andelectrode formed on an upper surface of the ridge and a protective filmdisposed on each side of the ridge. The electrode includes a flatportion having a flat surface substantially parallel to the uppersurface of the ridge and sloped portions on both sides of the flatportion with each of the sloped portions having a sloped surface that issloped with respect to the upper surface of the ridge. The protectivefilm covers a region from a side surface of the ridge to the slopedsurface of the sloped portion of the electrode.

Also, it is preferable that the semiconductor device described abovefurther includes at least one described below.

(1) An uppermost end of the protective film is positioned higher thanthe upper surface of the electrode.

(2) The uppermost end of the protective film is about 100 nm or morehigher than the upper surface of the electrode.

(3) The electrode is bonded with the semiconductor layer only at theupper surface of the ridge.

(4) A recessed shape is defined at the bonding portion between theprotective film and the electrode.

(5) A recessed shape is defined between the end portion of theprotective film and the upper surface of the electrode.

(6) A pad electrode is provided on the electrode and on the protectivefilm.

A method of manufacturing a semiconductor device according to a secondaspect of the present invention includes: forming a ridge on asemiconductor layer stacked on a substrate by forming a first mask layerhaving a predetermined shape on the semiconductor layer and removing apart of the semiconductor layer from an opening of the first mask layer;forming a second mask layer on a region from at least a bottom surfaceregion of the ridge to the first mask layer on an upper surface of theridge; removing a part of the second mask layer on the upper surface ofthe ridge to define an opening having a diameter smaller than a width ofthe ridge; removing the first mask layer on the upper surface of theridge to expose the upper surface of the ridge; forming an electrode onthe ridge so as to have a flat portion having a flat surfacesubstantially parallel to the upper surface of the ridge and slopedportions on both sides of the flat portion with each of the slopedportions having a sloped surface that is sloped with respect to theupper surface of the ridge; removing the second mask layer; forming athird mask layer on the flat surface of the flat portion of theelectrode; forming a protective film on a region from at least thebottom surface region of the ridge to the third mask layer on the uppersurface of the ridge; removing the third mask layer and a part of theprotective film on the upper surface of the ridge to expose theelectrode; and forming a pad electrode at least on an upper surface ofthe electrode and the protective film.

Also, it is preferable that the method of manufacturing a semiconductordevice described above further includes at least one described below.

(1) The removing of the third mask layer and the part of the protectivefilm includes removing the part of the protective film so that anuppermost end position of the protective film is higher than the uppersurface of the electrode and so that the protective film is disposed oneach side of the ridge to cover a region from the side surface of theridge to the sloped surface of the sloped portion of the electrode.

(2) The forming of the pad electrode includes forming the pad electrodeon the electrode through a conductive layer.

A semiconductor device according to the present invention is capable ofpreventing light absorption by an electrode, so that a semiconductordevice with improved reliability can be obtained.

Also, according to a manufacturing method of a semiconductor deviceaccording to the present invention, etch-back step can be omitted, sothat yield can also be improved.

Also, a wider range of materials can be used for the protective film andelectrodes, so that an expensive material is no longer needed to beselected.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of thisoriginal disclosure:

FIG. 1 is a schematic cross sectional view illustrating a structure of asemiconductor device according to an embodiment of the presentinvention.

FIG. 2 a is a partial schematic cross sectional view of a structure of asemiconductor device according to an embodiment of the presentinvention.

FIG. 2 b is a partial schematic cross sectional view of a structure of asemiconductor device according to an embodiment of the presentinvention.

FIG. 2 c is a partial schematic cross sectional view of a structure of asemiconductor device according to an embodiment of the presentinvention.

FIG. 2 d is a partial schematic cross sectional view of a structure of asemiconductor device according to an embodiment of the presentinvention.

FIGS. 3( a) to (d) are schematic cross-sectional views illustrating amethod of manufacturing a semiconductor device according to the presentinvention.

FIGS. 4( e) to 4(g) are schematic cross-sectional step views followingFIGS. 3( a) to 3(d).

FIGS. 5( h) to 5(j) are schematic cross-sectional step views followingFIGS. 4( a) to 3(d).

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Preferred embodiments of the semiconductor device and the method ofmanufacturing the semiconductor device according to the presentinvention will now be described below with reference to the accompanyingdrawings.

A semiconductor device according to the present invention includesmainly, a substrate, a semiconductor layer, an electrode, and aprotective film. Such a semiconductor device typically has, as shown inFIG. 1, a semiconductor layer 20 made of an n-side semiconductor layer11, an active layer 12, and a p-side semiconductor layer 13 stacked inthis order on a substrate 10, and a stripe-shaped ridge 14 is formed ona surface of the semiconductor layer 20. An electrode 15 is formed on anupper surface 14 c of the ridge. The electrode 15 has a flat portion 15a and sloped portions 15 b disposed on both sides of the flat portion 15a. The flat portion 15 a has a flat surface substantially parallel tothe upper surface 14 c of the ridge, and each of the sloped portions 15b has a sloped surface that is sloped with respect to the upper surface14 c of the ridge. A protective film 16 is formed on the bottom surfaceregion 14 a of the ridge and a side surface 14 b of the ridge, and theprotective film 16 further covers a sloped portion 15 b of the electrode15. As used herein a “side of the ridge” refers to a lateral side of theridge as viewed in a direction in which the stripe-shaped ridge extends(as seen in FIG. 1), and a “side surface of the ridge” refers to alateral side surface of the ridge that extends in the direction in whichthe stripe-shaped ridge extends. Also, as used herein, a “bottom surfaceregion of the ridge” refers to a region including an upper surface ofthe semiconductor layer that is disposed on a side of the ridge.

Herein, the cross-sectional shape of the electrode 15 formed on theupper surface 14 c of the ridge has a flat portion 15 a over the centerportion of the ridge and a sloped portion 15 b on the both sides of theflat portion, the sloped portion has a thickness decreasing from theends of the flat portion toward respective side surfaces 14 b of theridge. The width of the flat portion of the electrode is 1.8 μm to 25.0μm. The length of the sloped portion 15 b is the length from the end ofthe flat portion 15 a to the end of the upper surface of the ridge alongthe surface, and can be appropriately adjusted according to the width ofthe ridge and the width of the flat portion 15 a of the upper surface ofthe ridge, but the length of the sloped portion is preferably in a rangeof 0.1 μm to 3.0 μm. Herein, the width of the ridge is the width of theupper surface 14 c of the ridge. The sloped portion 15 of the electrodemay include a region with two or more steps and/or a parabolic shape.

The thickness of the electrode 15 is 100 nm to 500 nm. In thisspecification, the term “thickness of electrode” means the height fromthe upper surface 14 c of the ridge to the surface of the flat portion15 a of the electrode. The thickness of the sloped portion 15 b of theelectrode is the height from the upper surface 14 c of the ridge to thesurface of the sloped portion 15 b of the electrode, but the height mayvary depending on the measured position. The width of the electrode atthe upper surface 14 c of the ridge is approximately the same as thewidth of the ridge, but in a case where the width of the ridge is 7 μmor larger, the width of the electrode may be smaller than the width ofthe ridge. With this arrangement, absorption of laser beam by theelectrode can be prevented efficiently. The inclination θb of the slopedportion 15 b of the electrode is an angle between the upper surface 14 cof the ridge and a sloped portion 15 b, and can be appropriatelyadjusted according to the width of the ridge, the width of the flatportion of the electrode, and the thickness of the electrode, which isfor example about 10° to about 30°.

An uppermost end of the end portion 16 a of the protective film 16 whichis formed on the side surface 14 b of the ridge of the semiconductordevice is positioned higher than the upper portion 14 c of the ridge,and also covers the sloped portion 15 b of the electrode 15. Herein, theprotective film 16 does not cover the flat portion 15 a of the electrode15 on the upper surface of the ridge, so that a contact area between theelectrode 15 and the pad electrode 18 can be secured. Also, depending onthe length of the sloped portion 15 b of the electrode, the end portion16 a of the protective film is not necessarily cover the entire surfaceof the sloped portion 15 b of the electrode, as long as the end portionof the protective film covers 80% or larger area of the surface of thesloped portion 15 b of the electrode at least at one side. Absorption oflaser beam by the electrode can be prevented more efficiently with theuppermost end of the end portion 16 a of the protective film 16 arrangedat a higher position than the flat portion 15 a of the electrode 15.

In particular, in manufacturing a single mode semiconductor laserdevice, even if the width of the electrode formed on the ridge isadjusted, if the width of the ridge is wide, the laser beam emitted fromthe laser device becomes multimode. If the application of thesemiconductor laser device allows employing beam of multimode type, thelaser beam being multimode may not be a problem, but the width of theelectrode increases in proportion to the width of the ridge, theresulting absorption of laser beam by the electrode will be moreproblematic. The above described arrangements of the present inventionallow efficient prevention of absorption of laser beam at the electrodeon the ridge even in such a semiconductor laser device of wide-ridgetype. To be specific, the length of the sloped portion 15 b of theelectrode is increased as the width of the ridge is increased.

On the other hand, merely reducing the width of the electrode on theridge may result in increasing in the operating voltage. With this, asemiconductor laser device of high reliability cannot be obtained. Asemiconductor device according to the present invention with thestructure described above enables to fully utilize the width of theridge, so that increase in the operating voltage can be prevented andfurther, stable lateral confinement of light can be achieved.

Further, such a semiconductor device includes a pad electrode 18 whichcovers the electrode 15 on the ridge 14 and the protective film 16.Moreover, the semiconductor device may have a second protective filmformed on a side surface of the semiconductor layer 20. Although notshown, the front-side end surface and/or the rear-side end surface, eachof which is a cavity end surface of the semiconductor device, isprovided with a protective film made of insulating oxide film or nitridefilm, for example. It is sufficient that the protective film 16 and thesecond protective film are made of an insulating material, which furtherpreferably has a low refractive index.

As shown in FIG. 1, an n-electrode 19 is formed on the back surface ofthe substrate. Alternately, an n-electrode 19 may be formed in contactwith the n-side semiconductor layer 11 on the semiconductor layer 20side of the substrate.

In the semiconductor device according to the present invention, theregion of the semiconductor layer to which the electrode 15 is connectedis only the upper surface 14 c of the ridge. Therefore, occurrence ofcurrent leakage due to the electrode being in contact with a sidesurface of the ridge can be avoided. The end portion 16 a of theprotective film 16 covers the sloped portion 15 b of the electrode,which enables to prevent the electrode detaching from the ridge. The padelectrode 18 is in contact with the bonding interface between theelectrode 15 and the protective film 16. The cross section of thebonding interface region between the electrode 15 and the protectivefilm 16 has a recessed shape with the upper surface of the electrode 15forming a bottom of the recessed shape and the end surfaces of theprotective film 16 forming side surfaces of the recessed shape,therefore adhesion between the pad electrode 18 and the electrode 15 canbe improved by forming the pad electrode in the recess. The padelectrode 18 also covers an outer surface of the protective film 16 aswell as the recess.

A method of manufacturing a semiconductor device according to oneembodiment of the present invention will be described below withreference to steps (a) to (j) shown in FIGS. 3( a) to 3(d), FIGS. 4( e)to 4(g), and FIGS. 5( h) to 5(j). In the method of manufacturing asemiconductor device, in step (a), firstly, a semiconductor layerincluding an active layer is formed on a substrate. The semiconductorlayer here is such that an n-side semiconductor layer, an active layer,and a p-side semiconductor layer are stacked on a substrate in thisorder.

The substrate is preferably sapphire, spinel (MgAl₂O₄), silicon carbide,silicon, ZnO, GaAs, or a nitride substrate (GaN, MN, and the like). Thethickness of the substrate may be for example, about 50 μm to 10 mm. Thenitride substrate may be formed by using a vapor-phase epitaxial methodsuch as MOVPE, MOCVD (Metalorganic Chemical Vapor Deposition), and HVPE(Halide Vapor Phase Epitaxy), a hydrothermal synthesis method in whichcrystal growth is performed in a supercritical fluid, a high-pressuremethod, a flux method, a fusion method, or the like. A commerciallyavailable nitride substrate may also be used. Such a substrate isfurther preferably, for example, a nitride substrate having an offsetangle of from about 0.03 to 10° in a first principal plane and/or asecond principal plane. It is sufficient that the number of dislocationper unit area is 1×10⁷/cm² or less.

The n-side and p-side semiconductor layers among the n-sidesemiconductor layer, the active layer, and the p-type semiconductorlayer are, for example, made of a group III-V nitride semiconductorlayer such as AlN, GaN, AlGaN, AlInGaN, and InN. Among those, a nitridesemiconductor layer containing Al is suitable. Specifically, a galliumnitride based compound semiconductor layer of In_(y)Al_(z)Ga_(1-y-z)N(0≦y, 0≦z, y+z≦1), particularly, Al_(x)Ga_(1-x)N (0<x<1) or the like ispreferable. Each of the semiconductor layers described above has asingle-layer structure or a stacked-layer structure. Alternatively, astructure having a superlattice structure may be employed.

The n-side semiconductor layer has a cladding layer, and further, anoptical guide layer or a crack-preventing layer may be provided betweenthe cladding layer and an active layer which is to be described later.An under layer or a crack-preventing layer may be provided between thesubstrate and the cladding layer. The p-side semiconductor layer has acladding layer and a contact layer, and a cap layer or an optical guidelayer may be provided between the active layer to be described later andthe cladding layer.

The n-side semiconductor layer and the p-side semiconductor layer can beformed by using a similar method as used in the nitride substrate. Then-side semiconductor layer is doped with an n-side impurity such as Sior Ge, and the p-side semiconductor layer is doped with a p-sideimpurity such as Mg or Zn to render respective conductivities. Thedoping concentration may be, for example, about 1×10¹⁶ to 5×10²⁰ cm⁻³.

The active layer may have either a multiquantum well structure or asingle quantum well structure. The thickness of the active layer issuitably, for example, about 10 nm to 500 nm. Particularly, in a casewhere a quantum well layer structure is employed, the thickness of thewell layer and the number of the well layers are not specificallylimited. For example, with the thickness in a range from about 1 to 30nm, Vf, the threshold current density can be reduced. It is preferablethat the thickness of the well layer is in a range of 10 nm or less toreduce the thickness of the active layer. The thickness of the barrierlayer is, for example, 100 nm or less, and preferably in a range of fromabout 1 nm to 75 nm. Although the range of the oscillation wavelength ofthe active layer is not specifically limited, in a case where a nitridesemiconductor layer is used, it is, for example, 350 nm or more and 650nm or less.

Next, a mask layer of a predetermined shape is formed on thesemiconductor layer of the wafer where the semiconductor layers arestacked on the substrate. A method of forming a mask layer will bedescribed below, but the method of forming a mask layer is not limitedthereto. Firstly, a mask layer 21 (one example of a first mask layer)and a resist layer are formed in this order on the semiconductor layer.The resist layer is patterned into a predetermined shape, and then usingthe resist layer as a mask, the mask layer is patterned into the sameshape as the resist layer. After the mask layer is patterned, the resistlayer is removed to obtain the mask layer having the predeterminedshape. Examples of the materials of the mask layer include SiO₂, SuON,SiN, and the like. The mask layer can be formed using any other knownmask material. The thickness of the mask layer is not specificallylimited and for example, a thickness of about 100 nm to 1000 nm issuitable, and about 200 nm to 600 nm is preferable. The mask layer canbe formed by using a known method such as a CVD method, a sputteringmethod, and a vapor deposition method.

Next, a ridge is formed on the semiconductor layer. A part of the p-sidesemiconductor layer at an upper surface of the semiconductor layer isremoved from the openings of the mask layer to form a ridge. The methodof removing a part of the p-side semiconductor layer is not specificallylimited and either wet etching or dry etching can be used. Specifically,in view of the materials of the semiconductor layer, it is preferablethat an etchant having a high etching selectivity to the mask layer isselected to remove the layer. The size of the ridge is approximatelyequal to the size of the mask layer, and the shape of the ridge may be aforward mesa shape having a width larger at the bottom surface side andbecoming smaller toward the upper surface, a shape having a side surfaceperpendicular to the stacked layer surface, or a combination of those.The width of the ridge is not specifically limited, but thesemiconductor devices can be manufactured without impairingmanufacturing yields, in a case where the width of the ridge is 100 μmor less. The width of the ridge is, for example, suitably about 2.0 μmto 30.0 μm, and preferably about 5.0 μm to 20.0 μm. The height of theridge can be appropriately adjusted by the thickness of the p-sidesemiconductor layer. For example, a height of about 0.1 μm to 2 μm,further about 0.2 μm to 1 μm may be employed.

Next, in step (b), a second mask layer 22 is formed at least in a regionfrom the bottom surface of the ridge to the mask layer 21 on the uppersurface of the ridge. The second mask layer 22 is formed not only on thebottom region 14 a of the ridge and the side surface 14 b of the ridgebut also on the region of the upper surface of the ridge where the masklayer 21 is formed. Herein, the material for the second mask is notspecifically limited, but it is preferable to form a resist layer in apredetermined pattern. The thickness of the second mask is notspecifically limited, and for example, about 0.1 μm to 3.0 μm may beemployed.

Next, in step (c), an opening is defined in the second mask layer 22.Herein, an opening is defined in the second mask layer 22 by usingphotolithography, dry etching, wet etching, or the like. The opening isdefined in the second mask layer 22 on the mask layer 21 in a desiredstripe shape. The width of the opening is appropriately adjustedaccording to the width of the ridge. For example, if the width of theridge is 100 μm or less, the width of the opening will be 99 μm or less.If the width of the ridge is 30 μm or less, the width of the openingwill be 1.0 μm to 29.0 μm.

For the method defining an opening portion in the second mask,photolithography may be used in view of the material of the resistpattern and the like, or selecting an appropriate etchant, either wetetching or dry etching may be used. Examples of an appropriate etchantinclude, a single acid such as nitric acid, hydrofluoric acid, dilutehydrochloric acid, dilute nitric acid, sulfuric acid, hydrochloric acid,acetic acid, and hydrogen peroxide, or a mixture of two or more suchacids, a single alkaline solution of ammonia or the like, a mixedsolution of ammonia and hydrogen peroxide or the like, and varioussurface active agents. For the method of removing the second maskremaining unnecessary portions, a known method such as immersion,ultrasonic treatment, or a combination thereof may be used. Thesemethods can also be used for removing other mask layers or a third masklayer.

Next, in step (d), the mask layer 21 is removed from the opening portionof the second mask layer 22. For the method of removing the mask layer21, etching such as wet etching is preferably used. For the method ofremoving the mask layer 21, the method used in defining the openingportion in the second mask layer 22, which is described above, can alsobe used. With this, the upper surface 14 c of the ridge is exposed.

Next in step (e), an electrode material film is formed on the uppersurface 14 c of the ridge and the second mask layer 22. Herein, theelectrode material film is formed on the upper surface of the ridgewhich is exposed after the removing step (d) and on the second masklayer. A known method can be used for the method of forming theelectrode material film, but a sputtering method, or a vapor-depositionmethod is preferably used. Herein, the flat portion 15 a and the slopedportions 15 b can be formed by forming the electrode 15 through theopening portion of the second mask 22. The width of the electrode whichis in contact with the upper surface of the ridge is approximately thesame as the width of the ridge. In a case where the width of the ridgeis 7.0 μm or larger, the electrode may be formed narrower than the widthof the ridge.

A material generally used for an electrode can be used as the electrodematerial. For example, a single film or stacked layer film of a metal oran alloy, conductive oxide film, or the like, may be employed. Thethickness of those electrode materials is suitably about 50 nm to 1000nm, and preferably about 100 nm to 500 nm. Specific examples include atwo-layer structure of Ni (thickness: about 5 to 20 nm) at thesemiconductor layer side and Au (thickness: about 50 to 300 nm), andNi—Au—Pt, Ni—Au—Rh, Ni—Au—RhO₂, Ni—Au—Pd, Ni—Au—Ir, Ni—Au—Ru, or thelike, including the two-layer structure. Examples thereof also includetwo layer structures such as Pd—Au, Pd—Pt, and Ni—Pt, and three layerstructures such as Ni—ITO—Pt, Ni—ITO—Rh, Pd—Pt—Au, Pd—Pt—Rh, Pd—Pt—Ir,and Rh—Ir—Pt. Those electrode material films can be formed by using aknown method such as a CVD method, a sputtering method, and a vapordeposition method. The thickness of the electrode material film is notspecifically limited, and for example, employing a thickness of about 50nm or more enables to reduce the sheet resistance.

Next, in step (f), the second mask layer 22 and the electrode material15 thereon are removed to expose the bottom surface region 14 a and theside surface 14 b of each side of the ridge. Herein, removing the secondmask layer 22 also removes the electrode material 15 thereon at the sametime. Any one of the methods described above can be used for removingthe second mask layer 22, but a liftoff method or a wet etching methodis preferably used.

Next, in step (g), a third mask layer 23 is formed on the flat portion15 a of the electrode. Herein, a resist layer is formed in a pattern asthe third mask 23. For the method of forming the third mask layer in apattern, photolithography is preferably used. The thickness of the thirdmask 23 is not specifically limited, and for example, a thickness ofabout 0.1 μm to 4.0 μm may be employed. The third mask layer 23 maycover only the entire upper surface of the flat portion 15 a of theelectrode, may cover the entire upper surface of the flat portion 15 aof the electrode and a part of the sloped portion 15 b of the electrode,or may cover only slightly smaller portion than the flat portion 15 a ofthe electrode.

Next, in step (h), a protective film 16 is formed to cover the bottomsurface regions 14 a of the ridge, the side surfaces of the ridge 14 b,the sloped portions of the electrode 15 b, and the third mask layer 23.The protective film 16 is formed by using a known method in the art suchas a sputtering method, a vacuum deposition method, or a vapor-phasegrowth method. The thickness of the protective film is, for example,3000 nm or less, preferably about 20 nm to 1000 nm, and suitably about50 nm to 500 nm.

The protective film 16 secures insulation between the upper surface ofthe p-side semiconductor layer, which is the bottom surface region 14 aof the ridge, and the side surfaces of the ridge, and also secures thedifference in refractive index between the p-side semiconductor layerand the protective film 16, thus, capable of controlling the leakage oflight from the active layer. Also, the semiconductor device according tothe embodiment has an end portion 16 a of the protective film formed ina latter step, thus absorption of light at the upper portion of theridge can be prevented. The material of the protective film is notspecifically limited as long as the material has insulation property.Examples thereof include Si, Mg, Al, Hf, Nb, Zr, Sc, Ta, Ga, Zn, Y, B,Ti, and chemical compounds thereof, such as oxides, nitrides (forexample, AlN, AlGaN, BN and so on) and fluorides. The protective filmmay be a single film or a multilayer film of a combination of aplurality of materials described above. Among those, a film made of amaterial having low refractivity and low absorption property such asSiO₂ and Al₂O₃ is preferable.

Next, in step (i), the third mask layer 23 and the protective film 16are removed from the upper surface of the ridge. By removing theprotective film which is located over the upper surface of the ridge,the uppermost end of the end portion 16 a of the protective film ispositioned higher than the ridge. With this, the upper surface 14 c ofthe ridge is exposed. The sloped portions of the electrode on the ridgeare covered with the protective film. In step (i), the end portion 16 aof the protective film may entirely cover the sloped portions 15 b ofthe electrode, or may partially cover the sloped portions 15 b of theelectrode with a part of the sloped portion 15 b being exposed. Further,it is preferable that if the uppermost end of the end portion 16 a ofthe protective film is positioned higher than the upper portion of theelectrode, that is, if it is at higher position than the surface of theflat portion 15 a of the electrode, the light absorption by the padelectrode can also be prevented efficiently. It is further preferablethat if the upper most end of the end portion 16 a of the protectivefilm is positioned 100 nm or more higher than the surface of the flatportion 15 a of the electrode which is the upper surface of theelectrode, the light absorption by the pad electrode can be preventedmore efficiently.

The end portion 16 a of the protective film is formed at this state byremoving the mask layer and the protective film provided above the uppersurface of the ridge. Therefore, the end portion 16 a of the protectivefilm is formed on each side of the ridge so as to continuously extend ina region from the side surface 14 b of the ridge toward the uppersurface of the ridge. The height of the uppermost end of the end portion16 a of the protective film can be adjusted by the thickness of thethird mask layer. The height of the uppermost end of the end portion 16a of the protective film is preferably in a range of about 50 nm to 1000nm from the upper surface 14 c of the ridge. Also, the height of theuppermost end of the end portion 16 a of the protective film is morepreferably 100 nm or higher than the upper surface of the electrode. Ifthe height of the uppermost end of the end portion 16 a of theprotective film is in this range, light absorption at the upper surfaceof the ridge can be prevented. Also, a recessed shape is defined in thebonding region of the protective film and the electrode with the uppersurface of the electrode forming a bottom of the recessed shape and theend surfaces of the protective film forming side surfaces of therecessed shape, thus adhesion between the electrode 15 and the padelectrode 18 can be improved. If the bonding region with the electrodeis arranged at the end portion 16 a of the protective film, a groove ofrecess portion can be easily formed.

Herein, the method of removing the third mask layer 23 and theprotective film formed on the third mask layer 23 is not specificallylimited, and a liftoff method can be used, for example. The conditionsof liftoff can be appropriately selected by, for example, the materialsetc. of the mask layer and protective film. For example, it is suitableto use an appropriate etchant such as a single acid or a mixture of twoor more acids of nitric acid, hydrofluoric acid, sulfuric acid,hydrochloric acid, acetic acid, and hydrogen peroxide, or a singlealkaline solution of ammonia or the like, a mixed solution of ammoniaand hydrogen peroxide or the like, and various surface active agents.Also, a known method such as immersion, rinsing, ultrasonic treatment,or a combination thereof may be used.

Further, in any step after step (i), as step (j), a pad electrode 18 isformed on the semiconductor device, on the protective film 16 and theelectrode 15. The pad electrode is preferably a stacked layer film madeof metals such as Ni, Ti, Au, Pt, Pd, and W. Specific examples thereofinclude W—Pd—Au or Ni—Ti—Au, Ni—Pd—Au formed in this order from thep-electrode side. The thickness of the pad electrode is not specificallylimited, but it is preferable that the thickness of Au which is thefinal layer is about 100 nm or more. The shape of the pad electrode isnot specifically limited.

In a method of manufacturing a semiconductor device according to theembodiment, it is preferable to polish a second principal surface of thesubstrate at any appropriate stage, for example, before forming then-side electrode. Any known method in the art can be used for polishingthe substrate.

Further, the n-side electrode is preferably formed on a part or entiresurface of the second principal surface of the substrate, before orafter forming the p-side electrode described above. The n-side electrodecan be formed by using, for example, a sputtering method, a CVD method,an evaporation method, or the like. A liftoff method is preferably usedfor forming the n-side electrode in a pattern, and after forming then-side electrode, annealing is preferably carried out at about 300° C.or higher. For the n-side electrode, for example, it is sufficient thatthe total thickness is about 1 μm or less. The material of the n-sideelectrode is not specifically limited, and for example, V (thickness of10 nm)—Pt (thickness of 200 nm)—Au (thickness of 300 nm) are stacked inthis order from the substrate side to form the n-side electrode. Otherexamples thereof include Ti (15 nm)—Pt (200 nm)—Au (300 nm), Ti (10nm)—Al (500 nm), Ti (6 nm)—Pt (100 nm)—Au (300 nm), Ti (6 nm)—Mo (50nm)—Pt (100 nm)—Au (210 nm), or the like.

A metallization electrode may be formed on the n-side electrode. Themetallization electrode can be made of, for example, Ti—Pt—Au—(Au/Sn),Ti—Pt—Au—(Au/Si), Ti—Pt—Au—(Au/Ge), Ti—Pt—Au—In, Au—Sn, In, Au—Si,Au—Ge, and the like. The thickness of the metallization electrode is notspecifically limited. In a case where ohmic property is maintained onlyby the metalizing electrode, the n-side electrode can be omitted.

Optionally, for example, a second protective film may be formed on theprotective film 16 after step (i). The second protective film can beformed by using any known method in the art, and the materials thereofcan be selected from the same materials as the above described materialsof the protective film.

Next, a cavity end surface is formed on the semiconductor layer. Thecavity end surface can be formed by using any known method in the artsuch as etching, cleaving, or the like. Also, a dielectric film ispreferably formed in an appropriate stage on the obtained cavity endsurface, that is, on the light-reflecting side and/or light emittingside of the cavity end surface. The dielectric film is preferably asingle-layer film or a multi-layer film made of SiO₂, ZrO₂, TiO₂, Al₂O₃,Nb₂O₅, AlN, AlGaN, or the like. Further, by dividing in the cavitydirection (resonating direction), semiconductor device chips can beobtained. The dividing can be carried out by forming a guiding groove inan appropriate stage and scribing, for example.

According to the method of manufacturing the semiconductor device of theembodiment, a semiconductor device of high reliability can be formed bysimplified steps without limitation of the material of the protectivefilm and electrodes. That is, an etch-back step, which is difficult tobe controlled in a typical semiconductor process, can be eliminated, andthus enables controlling of each steps with high accuracy. Therefore themanufacturing yield of a semiconductor device can be improved withsimplified steps. Also, according to the method of manufacturing of theembodiment, mass production efficiency of a semiconductor device can beimproved.

Examples of the semiconductor device according to the present inventionwill be described below, but the present invention is not limitedthereto. The semiconductor device herein will be exemplified with asemiconductor laser device.

Example 1

A semiconductor laser device of the present example includes, as shownin FIG. 1, a semiconductor layer of an n-side semiconductor layer 11, anactive layer 12, and a p-side semiconductor layer 13 stacked in thisorder is formed on a GaN substrate 10 having a C-plane as a growthsurface, and a ridge 14 is formed on the surface of the p-sidesemiconductor layer 13. A p-side electrode 15 is ohmically connected tothe upper surface of the ridge 14. The electrode 15 has a flat portion15 a and sloped portions 15 b. Each sloped portion 15 b of the electrodeis covered with the end portion 16 a of the protective film and a padelectrode is electrically connected to the flat portion 15 a of theelectrode 15.

Further, although not shown, a dielectric film made of Al₂O₃ is formedon the cavity end surface of the semiconductor layer. Also, a p-side padelectrode 18 is formed to cover the electrode 15 and the protective film16.

Such a semiconductor laser device can be formed according to a method ofmanufacturing described below.

Formation of Ridge

Firstly, a GaN substrate 10 is prepared. Next, on the substrate 10, asemiconductor layer 20 of an n-side semiconductor layer 11, an activelayer 12, and a p-side semiconductor layer 13 stacked in this order isformed. Then, a mask layer 21 of SiO₂ film is formed with a thickness of500 nm on the approximately entire surface of the p-side semiconductorlayer 13 by using a CVD apparatus. Thereafter, a pattern with a width of15.0 μm is formed in the mask layer by way of etching using an RIE(Reactive Ion Etching) apparatus. In this stage, the width of the masklayer is adjusted to the width of the ridge which will be describedlater.

Next, as shown in FIG. 3( a), a ridge is formed on the surface of thestacked semiconductor layer 20. Specifically, the ridge 14 is formed onthe surface of the p-side semiconductor layer 13 which is an upper layerof the semiconductor layer 20. Here, using an RIE apparatus, etching iscarried out on the p-side semiconductor layer 13 which is exposed at theopening of the mask layer 21 to form a stripe-shaped ridge 14 of about15.0 μm in width and about 0.8 μm in height.

Thereafter, as shown in FIG. 3( b), a second mask layer 22 is formed tocover from the bottom surface region 14 a of the ridge to the uppersurface of the mask layer 21. The second mask layer is formed with aresist layer with a thickness of 1.2 μm.

Next, as shown in FIG. 3 (c), an opening is defined in the second masklayer 22. The opening is formed by performing etching on the second masklayer 22 using an RIE apparatus. The etching is performed on the secondmask layer 22 over the mask layer 21 with the width of 13.0 μm in thestripe direction of the ridge. The surface of the mask layer is exposedby this etching.

Next, as shown in FIG. 3( d), the mask layer 21 exposed in the previousstep is removed by wet etching. With this, the upper surface 14 c of theridge is exposed.

Formation of Electrode

Next, as shown in FIG. 4 (e), an electrode 15 is formed on the ridge 14and the second mask layer 22. The electrode 15 is formed on the ridgethrough the opening of the second mask layer. Therefore, a flat portion15 a is formed on the center portion of the ridge and a sloped portion15 b is formed on each of the both sides of the center portion of theridge. The electrode may be formed with the material of Ni—Au—Pt in thisorder from the upper surface 14 c of the ridge. Here, the width of theflat portion 15 a of the electrode 15 is 13.0 μm and the width of thesloped portion 15 b formed each of the both sides of the flat portion is1.0 μm. Here, the thickness of the electrode is the thickness of theflat portion 15 a, and a total thickness is 210 nm with Ni (10 nm)—Au(100 nm)—Pt (100 nm) formed in this order.

Next, as shown in FIG. 4 (f), removing the second mask layer 22 alsoremoves the electrode formed on the second mask layer 22. The removal ofthe second mask layer is carried out employing a lift-off method inwhich a release solution is used. Through this method, the electrode onthe second mask layer is also removed together with the second masklayer. The electrode 15 is in contact with the semiconductor layer onlyon the upper surface 14 c of the ridge, so that current leakage does notoccur.

Next, as shown in FIG. 4 (g), a third mask layer 23 is formed on theflat portion 15 a of the electrode formed on the ridge 14. A resistlayer id used as the third mask layer 23. The width of the third masklayer 23 is about 13.0 μm and the thickness of the third mask layer isabout 2.5 μm.

Formation of Protective Film

Thereafter, as shown in FIG. 5 (h), a protective film 16 is formed tocover the bottom surface regions 14 a of the ridge, the side surfaces 14b of the ridge, the sloped portions 15 b of the electrode, and the uppersurface of the third mask layer 23. The protective film 16 is a SiO₂film with a thickness of 200 nm formed by using a sputtering apparatus.

Next, the third mask layer 23 formed on the electrode and the protectivefilm 16 formed on the third mask layer 23 are removed. A lift-off methodis used to remove the third mask layer. This exposes the flat portion 15a of the electrode. Also, an uppermost end of an end portion 16 a of theprotective film which is extended to a position higher than the uppersurface of the ridge is formed in the protective film 16 which coverseach side surface of the ridge. In this example, the height of anuppermost end of each end portion 16 a of the protective film is 500 nmfrom the upper surface 14 c of the ridge. With the uppermost end of theend portions 16 a of the protective film being higher than the uppersurface of the ridge with this amount, absorption of laser beam by theelectrodes around the upper surface of the ridge and the side surfacesof the ridge can be prevented. Although only the flat portion 15 a ofthe electrode is exposed in FIG. 5 (i), a part of the sloped portion 15b of the electrode may also be exposed. Also, the end portion 16 a ofthe protective film may cover a part of the flat portion 15 a of theelectrode.

Next, a second protective film is formed on the bottom surface region 14a of the ridge, in a region spaced apart by about 35 μm from a sidesurface 14 b of the ridge. The second protective film further covers theside surfaces of the semiconductor layer. The second protective film isnot shown in the figure. Thereafter, as shown in FIG. 5 (j), a p-sidepad electrode 18 is formed on the electrode 15 and the protective film16. Also, an n-side electrode 19 is formed on the back surface of thesubstrate 10. On the cavity end surface at the front side of thesemiconductor layer, a dielectric film made of Al₂O₃ is formed. On thecavity end surface at the back side of the semiconductor layer, adielectric multilayer film made of ZrO₂ and SiO₂ is formed. In thismanner, a semiconductor device can be formed.

As described above, in Example 1, the electrode is connected only at theupper surface of the ridge, so that leakage of current due to theelectrode comes in contact with a side surface of the ridge can beprevented. Also, a sloped portion is provided on the electrode and aprotective film made of SiO₂ which is a material having a lowrefractivity is applied on the sloped portion. Thus, absorption of lightby the electrode can be prevented and the extraction efficiency of laserbeam can be improved. Further, with the manufacturing steps as describedabove, an etch-back step is no longer necessary, which facilitatesmanufacturing of the semiconductor devices with stable properties.

Example 2

The semiconductor laser device of the present example is formed suchthat the third mask layer 23 is formed to cover the flat portion and apart of the sloped portion of the electrode in the manufacturing stepsdescribed in Example 1. Specifically, the width of the third mask layer23 is about 14.0 μm and the thickness of the third mask layer is about2.5 μm. Except as described above, the semiconductor laser device ismanufactured in substantially the same manner as the semiconductor laserdevice of Example 1. Accordingly, as shown in FIG. 2 b, a region coveredby the tip end portion 16 a of the protective film and a regionconnected to the p-side pad electrode are formed on each sloped portionof the electrode. Substantially the same effects as in Example 1 canalso be obtained in this example.

Example 3

The semiconductor laser device of the present example is formed suchthat the third mask layer 23 is formed to cover only the flat portion ofthe electrode in the manufacturing steps described in Example 1. Thewidth of the third mask layer 23 is about 12.0 μm and the thickness ofthe third mask layer is about 2.5 μm. Except as described above, thesemiconductor laser device is manufactured in substantially the samemanner as the semiconductor laser device of Example 1. Accordingly, asshown in FIG. 2 c, the tip end 16 a of the protective film covers thesloped portions 15 b of the electrode and a part of the flat portion 15a of the electrode. Effects substantially the same as that in Example 1can also be obtained in this example.

Example 4

The semiconductor laser device of the present example is formed asdescribed in Example 1 except that the electrode on the upper surface 14c of the ridge is formed so as not to entirely cover the upper surface14 c of the ridge and to expose a part of upper surface. Specifically,the width of the opening of the second mask layer 22 is 10.0 μm. With awidth of about 12.0 μm, the third mask layer 23 is formed to cover onlythe flat portion of the electrode. Except as described above, thesemiconductor laser device is manufactured in substantially the samemanner as the semiconductor laser device of Example 1. Thus, as shown inFIG. 2 d, the end portion of the upper surface 14 c of the ridge iscovered with the protective film and further, the end portion 16 a ofthe protective film covers the sloped portions 15 b of the electrode.Effects substantially the same as that in Example 1 can also be obtainedin this example.

The semiconductor devices according to the present invention can be usedin light emitting elements such as semiconductor lasers and lightemitting diodes. Examples of the applications include, a light sourcefor lighting, displays, optical disk applications, optical communicationsystems, printers, optical exposure applications, various devices formeasurement, excitation light source for bio-specific applications

It is to be understood that although the present invention has beendescribed with regard to preferred embodiments thereof, various otherembodiments and variants may occur to those skilled in the art, whichare within the scope and spirit of the invention, and such otherembodiments and variants are intended to be covered by the followingclaims.

1. A semiconductor device comprising: a semiconductor layer stacked on asubstrate; a stripe-shaped ridge formed on a surface of thesemiconductor layer; an electrode formed on an upper surface of theridge, and including a flat portion having a flat surface substantiallyparallel to the upper surface of the ridge and sloped portions on bothsides of the flat portion with each of the sloped portions having asloped surface that is sloped with respect to the upper surface of theridge; and a protective film disposed on each side of the ridge to covera region from a side surface of the ridge to the sloped surface of thesloped portion of the electrode.
 2. The semiconductor device accordingto claim 1, wherein an uppermost end of the protective film ispositioned higher than an upper surface of the electrode.
 3. Thesemiconductor device according to claim 1, wherein an uppermost end ofthe protective film is positioned 100 nm or more higher than an uppersurface of the electrode.
 4. The semiconductor device according to claim1, wherein the electrode is bonded with the semiconductor layer only atthe upper surface of the ridge.
 5. The semiconductor device according toclaim 1, wherein the protective film and the electrode define a recessedshape in a bonding region between the protective film and the electrode.6. The semiconductor device according to claim 1, wherein an end portionof the protective film on each side of the ridge and the upper surfaceof the electrode define a recessed shape.
 7. The semiconductor deviceaccording to claim 1, further comprising a pad electrode formed over theelectrode and the protective film.
 8. A method of manufacturing asemiconductor device comprising: forming a ridge on a semiconductorlayer stacked on a substrate by forming a first mask layer having apredetermined shape on the semiconductor layer and removing a part ofthe semiconductor layer from an opening of the first mask layer; forminga second mask layer on a region from at least a bottom surface region ofthe ridge to the first mask layer on an upper surface of the ridge;removing a part of the second mask layer on the upper surface of theridge to define an opening having a diameter smaller than a width of theridge; removing the first mask layer on the upper surface of the ridgeto expose the upper surface of the ridge; forming an electrode on theridge so as to have a flat portion having a flat surface substantiallyparallel to the upper surface of the ridge and sloped portions on bothsides of the flat portion with each of the sloped portions having asloped surface that is sloped with respect to the upper surface of theridge; removing the second mask layer; forming a third mask layer on theflat surface of the flat portion of the electrode; forming a protectivefilm on a region from at least the bottom surface region of the ridge tothe third mask layer on the upper surface of the ridge; removing thethird mask layer and a part of the protective film on the upper surfaceof the ridge to expose the electrode; and forming a pad electrode atleast on an upper surface of the electrode and the protective film. 9.The method of manufacturing a semiconductor device according to claim 8,wherein the removing of the third mask layer and the part of theprotective film includes removing the part of the protective film sothat an uppermost end position of the protective film is higher than theupper surface of the electrode and so that the protective film isdisposed on each side of the ridge to cover a region from the sidesurface of the ridge to the sloped surface of the sloped portion of theelectrode.
 10. The method of manufacturing a semiconductor deviceaccording to claim 8, wherein the forming of the pad electrode includesforming the pad electrode on the electrode through a conductive layer.11. A method of manufacturing a semiconductor device comprising: forminga ridge on a semiconductor layer stacked on a substrate by removing apart of the semiconductor layer; forming an electrode on the ridge so asto have a flat portion having a flat surface substantially parallel tothe upper surface of the ridge and sloped portions on both sides of theflat portion with each of the sloped portions having a sloped surfacethat is sloped with respect to the upper surface of the ridge; forming aprotective film disposed on each side of the ridge to cover a regionfrom the side surface of the ridge to the sloped surface of the slopedportion of the electrode; and forming a pad electrode at least on anupper surface of the electrode and the protective film.
 12. The methodof manufacturing a semiconductor device according to claim 11, whereinthe forming of the protective film includes forming the protective filmso that an uppermost end position of the protective film is higher thanthe upper surface of the electrode.
 13. The method of manufacturing asemiconductor device according to claim 11, wherein the forming of thepad electrode includes forming the pad electrode on the electrodethrough a conductive layer.